PROMs conventionally comprise a plurality of lateral or vertical fuses wherein a predetermined number of fuses are located in a row coupled to a word line and another predetermined number of fuses are located in a column coupled to a bit line. In other words, each fuse is uniquely coupled in a combination of columns and rows.
When a fuse is intact (electrically closed), it represents one digital state, i.e., "1". When the fuse is programmed or "blown" (electrically open), it represents a different digital state, i.e., "0". PROMs are manufactured with the fuses in the closed state and are subsequently programmed by applying a relatively high voltage between selected word lines and bit lines to open specific fuses.
Vertical fuses typically comprise two back to back diodes having common anodes. Lateral fuses typically comprise a film of a material, i.e., nichrome, horizontally formed across a surface of the semiconductor chip. Memory circuits over time have become larger and more complex. The ability of manufacturers to fabricate these memory circuits as integrated circuits is difficult and it is not rare for errors to be made wherein the fuses and associated on chip circuitry such as input/output buffers, regulators, decoders, sense amplifiers, and multipliers will not function properly.
One previously known method of testing the functionality of the memory circuit was to manufacture test rows and test columns on the PROM, in addition to the normal rows and columns; program fuses between the test rows and test columns; and then test for fuses actually programmed. This would give some indication of the probability of the remaining fuses on the PROM of being programmed successfully and of the functionality of the associated circuitry.
However, this method did not check all functional blocks of the circuit and it required additional pins to access test bits. If additional pins were not available, the test bits could not be used in packaged form.
Another previously known method of testing the functionality of the memory circuit was to program a sample of a group of PROMs; test the programmed sample for fuses actually programmed; and predict the functionality of the remaining PROMs from the group based on statistical probabilities.
However, this method destructively consumed the number of PROMs used in the sample and the results from the testing of the sample did not always correlate with the actual functionality of the remaining group.
Thus, what is needed is an on-chip circuit that provides for the testing of the functionality of the fuses and associated circuits of a memory circuit without requiring additional input/output pins.